Depends on the bootcamp, I go to a really effective one but have also been to a really crap one - as always the quality of the trainer is key, often the first trial session is free. I run but I also try to go to bootcamp twice a week as it really boosts my overall fitness & strength. I don't ski but our bootcamp trainer ran sessons last year tailored to getting ski fit, so that might be worth looking into.
I am sure a decent boot camp would provide a great boost to kick start weight loss and fitness but one weekend (or even a week) in itself won't be enough to make a dramatic difference. If I were you I would spend the money on a personal trainer once a week for 10-12 weeks (and train 2-3 times a week in addition to that). He/she would specifically tailor a programme to get you as ski fit as possible and I am sure it will result in fat loss too.
Just realised I only have a couple of months to go before I go skiing for the first time in a decade and am trying to work out the best way to improve my fitness in a short time. I need to build muscle strength and aerobic endurance - and if I could shift a few pounds at the same time, that would be fantastic. I am thinking of giving one of the miltary-style bootcamps a go and just wondered whether anyone here had any experience of that type of class and how effective it was for improving fitness (and losing flab)? Or is there a better way to improve fitness fast?